1. Field of the Invention
The present invention relates to a sheet conveyor having a conveyor belt that conveys a sheet material while holding the sheet material by attraction with an electric force, and to an image formation device equipped with such a sheet conveyor.
2. Description of the Related Art
Generally, image formation devices, such as printers, copiers, and facsimiles, form images of dot patterns on sheet materials (recording media), such as paper and thin plastic sheets, on the basis of image information. There are various recording types of image formation devices, some of which are, for example, an inkjet type, a wire-dot type, a thermal type, and a laser-beam emission type. In an inkjet type, an image is formed by discharging ink towards a sheet material, such as recording paper, from a recording head. An inkjet image formation device is characterized in having a compact recording head, having the capability to record high-resolution images at high speed, and requiring low operating costs. Furthermore, due to performing a recording operation in a non-impact fashion, an inkjet image formation device does not produce much noise. Moreover, an inkjet image formation device is also advantageous in having an ability to record color images readily using multiple colors. Of image formation devices of inkjet types, a full-line recording device equipped with a line-type recording head having a plurality of discharge nozzles arranged in a width direction of a sheet material achieves higher speed for recording.
In an image formation device, a sheet material fed from a paper-feeder unit, such as a cassette, has to be conveyed through an image-forming unit (recording unit) and then to a paper-ejector unit. In this case, the conveying operation for the sheet material is constantly controlled at a predetermined timing for the feeding step, the image formation step, and the ejection step. The conveying process from the feeding step to the image formation step especially requires accuracy since it directly affects the image forming position on the sheet material. Furthermore, if the conveying speed of the sheet material is not constant during the image formation step, the magnification of the image may vary, causing the image to expand or contract. Especially in an image formation device equipped with a plurality of recording heads for multiple colors, the images recorded by the recording heads (image-forming units) may become misaligned with one another. In a color image formation device, such misalignment will directly lead to color misalignment, resulting in a defective image. In order to prevent this, it is necessary to properly transmit a conveying force to the sheet material from a conveying unit that is controlled with high precision.
In view of these circumstances, a sheet conveyor having an endless belt is disclosed in which the endless belt conveys a sheet material while holding the sheet material securely by electrostatic attraction. If such a belt-type sheet conveyor that applies electrostatic attraction is used in, for example, a color image formation device equipped with a plurality of recording heads (image-forming unit), the conveying speed of the belt must be maintained precisely so that the image forming position of each recording head is accurate. In addition, the sheet material must be securely attached to a conveying member (such as a belt or a drum) so as to prevent the sheet material from being displaced or from floating.
However, in an image formation device having a plurality of full-line recording heads extending longitudinally in a direction crosswise to the conveying direction, such as a color image formation device, the recording head at the uppermost-stream position and the recording head at the lowermost-stream position in the conveying direction are separated from each other by a large distance. This can lead to flapping of the sheet material in the recording region, thus causing, for example, blurred images or paper jams. In a known technique for restricting a sheet material from floating, the sheet material is biased downward by means of electrodes provided in a conveyor belt. In detail, a voltage is applied to the electrodes to produce an electric force, by which the sheet material is attracted to the conveyor belt. Furthermore, a technique for attracting a sheet material to a conveyor belt by static electrification is also known. Moreover, in another known technique, a pressure control chamber is provided, and a fan is used to control the pressure so as to attract a sheet material to a conveyor belt by suction.
In a sheet conveyor that attracts a sheet material to a conveyor belt with an electrostatic force generated by applying an electric charge to electrodes (attraction-generating unit) provided in the conveyor belt as mentioned above, the sheet material fed from the paper-feeder unit is conveyed to the recording region of the recording heads while being attracted to the conveyor belt by the attraction-generating unit (i.e. the conductive electrodes). In the recording region, the recording heads form images on the sheet material. A typical structure of such a sheet conveyor is discussed in Japanese Patent Laid-Open Nos. 2000-247476 and 2000-60168.
Such related art has technical problems to be solved, which will be described below. FIG. 11 schematically illustrates the disadvantages in the sheet conveyor having the conveyor belt of the related art. With respect to image formation devices that hold a sheet material by electrostatic attraction and perform a non-contact recording operation as in an inkjet recording type, the technology for high-resolution recording is rapidly developing due to high-density packaging of recording heads. On the other hand, the dimension of recording heads is also increasing. Moreover, an improvement in gradation by increasing the installation number of recording heads is also in demand. Therefore, the flatness accuracy of sheet materials serving as recording media needs to be maintained not only in a region directly below each recording head but also over a wide region.
However, in the related art as shown in FIG. 11, if a platen that faces the recording heads is increased dimensionally in the conveying direction, the conveyor belt wound around the rollers with a certain tension undulates in a direction crosswise to the conveying direction. When the undulated conveyor belt slides on the platen, the undulation may cause air to enter the space between the two even if the conveyor belt is attached to the platen by means of an attractive force produced therebetween. This creates unattached sections in the conveyor belt, causing the conveyor belt to swell. Although such swelling of the conveyor belt can be reduced by lowering the tension, it is difficult to achieve high-accuracy conveying in that case since the conveyor belt may be subject to slipping against the rollers.